Canine monocytic ehrlichiosis (CME) is a severe rickettsial disease in dogs caused by the bacteria Ehrlichia canis (E. canis). E. canis is spread from dog to dog via the Brown Dog Tick (Rhipicephalus sanguineus). Following an incubation period of 2 to 3 weeks post-infection, dogs may progress through acute, subclinical, and chronic phases of the disease. In the acute phase, clinical signs range from mild to severe thrombocytopenia, leukopenia, anemia, lethargy, and weight loss. After two months, most dogs will enter a subclinical phase lasting months or years, during which low blood values may persist, but clinical signs are minimal. A small percentage of infected dogs may develop a severe form of the disease known as Tropical Canine Pancytopenia (TCP). Persistent bone marrow depression, hemorrhages, neurological disturbances, peripheral edema, and severe weight loss are characteristic of TCP. Hypotensive shock may develop, leading to death. Despite reports of immunogenic compositions against E. canis [see, e.g., Mahan et al., Onderstepoort J. Vet. Res. 72(2): 119-128 (2005); US 2006/0188524A1] heretofore, there has been no demonstration of an effective vaccine against E. canis. Therefore, there is a need for vaccines that protect against CME and/or TCP.
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